Method for making cbe-on-face magnetic steel



Patented. June 14, 1960 ICC METHOD FOR MAKING CUBE-ON-FACE. MAGNETIC STEEL John H. Hollomon, Schenectady, N.Y., assignor to General Electric Company, a corporation of New York N Drawing. Filed Sept. 20, 1956, Ser. No. 610,904 1 Claim. (Cl. 148-111) This invention relates to magnetically sof, polycrystalline, body-centered cubic alloys and more particularly to the fabrication of such polycrystalline sheet materials composed of iron, iron and silicon, iron and aluminum, and iron andv molybdenum having cube texture grain orientation.

As pointed out and particularly disclosed in copending application for United States Letters Patent Serial No. 610,909 of Hibbard and Walter, filed concurrently with this application and assigned to the same assignee, certain alloys of iron have been found to have very desirable magnetic and electrical properties. These materials have been Widely used in the form. of laminated sheet metal cores and the like for saturable core devices, motors, generators, and other electromagnetic apparatus. The alloys which have the most desirable and useful electrical and magnetic properties have been composed of up to about 5% silicon, balance substantially all iron, up to about 8% aluminum, balance substantially all iron, and up to about 5% molybdenum, balance substantially all iron.

Additionally, it has been found that for certain purposes a strong preferred grain orientation is desirable in sheet and strip material formed by rolling or forging, particularly in the case of silicon steel sheet or strip. This preferred grain orientation has been previously produced in silicon steel, for example, by hot and cold rolling silicon steel ingots into sheets or strips combined with appropriate heat treatment, as is well known in the art. The finished product is, of course, a polycrystalline material which has the body-centered cubic lattice form, and in which a majority of the grains have substantially the same orientation with respect to the plane ofthe sheetor strip and the direction of rolling. This orientation may be described as one in which the unit cube lattices of the oriented grains have a plane containing diagonally opposite cube edges substantially parallel to the plane of. the. sheet and a patr ot opposite cube faces substantially perpendicular to the rolling direction and to the plane of the sheet. This orientation is conventionally defined and described by metallurgists and crystallographers as the (110) [001] orientation in terms of the Miller Crystallographic Index System, a complete discussion of which may be found, for example, in Stmcture of Metals, C. S. Barrett, 2nd Edition, 1952, the Macmillan Company, pages 1 to 25.

As is well known in the art. and more particularly set forth in application Serial No. 610,909, previously referenced, this preferred orientation improves the useful magnetic properties of these materials in the plane of the sheet and in the rolling direction, but these magnetic properties in the plane of the sheet and transverse to the rolling. direction are comparativelyp'oor.

As disclosed in the previously referenced co'pending application, it has been found that by appropriately rolling and heat treating silicon-iron, molybdenum-iron and aluminum-iron grain oriented castings, a different preferred orientation may. be produced in polycrystalline sheet or strip materials of these compositions. orientation may be described as one in which a majority of the grains have their body-centered cubic lattices oriented: so that four (4) of the cube faces are substantially parallel to the rolling direction, two of these faces being also substantially parallel with the plane of the sheet and the other two being substantially perpendicular .to the plane of the sheet, and the remaining .two cube faces substantially perpendicular to both the rolling direction and the plane of. the sheet. This orientation may be conveniently termed cube texture or defined in terms of Miller Indices, (100) [001]. These sheet and strip materials having, this orientation have been found to have magnetic properties which are equivalent to those of previously known (1'10) [001] materials ofv the same composition in the rolling direction and in the plane of the sheet, and much more desirable magnetic properties in the plane of the sheet transverse to the rolling direction than the previously known. materials.

As disclosed in the previously reference application, this preferred orientation, i.e., cube texture, may be produced by particularly preparingcast ingots having an entirely or substantially entirely columnar as-cast grain structure. These castings are produced so that substan. tially all the metal therein is comprised of elongated columnar grains, the longitudinal axis of each of whichis: substantially parallel to each other longitudinal grain axis and all of which. axes are substantially parallel to a single direction in the ingot. It was found that byape propriately hot, warm and cold rolling as-cast slabs; of up to about 1 inch in thickness or slabs of that thickness cut from larger ingots, with particular reference to the? direction of the longitudinal axes of the columnar grains to the rolling directions and rolling planes that the cold: rolled structure of the sheet material so produced could be caused to recrystallize upon appropriate annealing to produce sheet orstrip material having the desired cube- .texture. More specifically, this reduction procedure involved heating such slabs to a temperature range of from. about 700 C. to about 1100" C., reducing the thickness of the so-heated slabs about to 97% in a plurality of rolling passes without reheating, annealing, cold reducing the annealing material at least 40% in thicknessby cold rolling and annealing to effect recrystallization of the cold rolled grain structure to produce the cube textured sheet or strip material. This casting, rolling. and heat treatment procedure was found to be eifective'to: produce cube texture in polycrystalline body-centered cubic sheet or strip materials composed of at least 92% iron and more specifically alloys containing up to about 5% silicon, up to about 5% molybdenum and up to about 8% aluminum, all useful soft magnetic materials.

Obviously, a rolling process which depends uponstart ing with as-cast slabs or slab ingots about 1 inch thickwould pose many problems in adaptation to conventional steel mill production procedures. It would be desirable to be able to process much thicker tions by rolling without having to therefrom.

Accordingly,

ingots or as-cast sec-- first prepare such slabs it is a principal object of my invention to provide a process whereby columnar grain ingots or cast intermediates. or blanks composed of at least 92% iron and containing up to about 5% silicon or 5% molybdenum or 8% aluminum, and havingthicknesses substantiab 1y greater than 1 inch may be worked to thinner intermediate thicknesses having a recrystallized cube texture grain structure and subsequently worked to form sheet or strip material of conventional thicknesses and annealed to? finished: prodproduce a recrystallized cube texture in the uct.

An additional object of my invention is the provision sheet or strip material may be producedrfrom ingots com- 7 sectioned grain oriented castings of these posed of columnar as-cast grains without regard to the initial thicknessof the start ng flight t n av to perform a slabbing operation'upon' such ingots.

Othjer and specificallydifierent objects of my invention will become apparent from the following detailed disclosure. K V t Briefly stated, in accordance with one embodiment of my invention, 1 have discovered that the cubic texture in bodycentered alloys of iron comprising silicon-iron,

molybdenum-iron, and aluminum-iron which may be pro duced in wrought recrystallized sheet or strip-like bodies of these materials, unexpectedly may be retained through a successive working and recrystallization treatments so that there is no limitation save practical or economical considerations on the initial size of the ingot'which may be so processed. a

Previous investigators have found that polycrystalline iron and body-centered cubic iron alloys, such as, for example, commercial silicon-iron electrical grade alloys containing 2 to 5% silicon, less than about 0.005% carbon, balance substantially all iron, including minor amounts of impurities customarily present in such materials, tend to invariably assume certain well known preferred orientations? or texture when subjected to substantial amounts of cold plastic deformation by unidirectional rolling. The cold rolled texture of these materials is chiefly one in which [110] directions of substantially all the grains lie along the direction of rolling, with adeviation of a few degrees, and (001) planes liein the plane of the rolled sheet, with a deviation from this position chiefly about the rolling direction as an axis.

These'stable, cold rolled crystal orientations have been identified as (100) [011], (115) [110], (113) [110], The higher the amount of unidirectional cold reduction given such materials, the greater is the tendency of the grains to as- 7 sume a (100) [011] orientation in the rolledsheet. This maybest be described as an orientation in which a pair of opposite unit cube faces of the body-centered cubic lattices'of a majority of the grains in the sheet are sub- 7 stantially parallel to the'rolling direction and to the plane of the sheet and a'plane containing diagonally opposite cube edges of such unit cubes is substantiallyper'pendicular to theplane of the sheet and substantially parallel 'to' the rolling direction. Upon recrystallization by annealing such cold rolled sheets, the preferred orientation or texture of the annealed sheet will change to an orientation which jis different'from" the cold rolled texture. The 'annealed'preferred orientation of such materials has previously been found to consist of a majority of the grains havinga (110) [001] orientation, which has been previouslydescribed, It would, therefore, reasonably be expectedthat when polycrystalline bodies of iron and the body-centered alloys of iron were severely cold rolled in one direction into sheet material the grains comprising such bodies would assume the (100) [011] stable texture or one of the previouslymentioned variations thereof, and when recrystallized by annealing, that the annealed texture would be predominantly the (110) [001] orientation usually obtained. I have discovered, however, that contrary to that which would be expected, the plastically deformed structure resulting from unidirectionally cold rolling cube texture polycrystalline iron base alloys of this type recrystallizes into cube texture instead of a predominantly (110) [001] orientation.

'-Further, I have discovered that once a strong cube texture is obtained in such polycrystalline materials, it may cold rolled and annealed according to my invention several'times in succession, and will still retain the cube texture upon recrystallization after each successive recrystallization anneal. i

More specifically, according to my invention,iheavy "ished'material from grain oriented ingots of large size by rolling and heattreating. uAs illustrative of my invention, a grain oriented casting composed of about 3% by weight silicon, balance substantially all iron was prepared by pouring the molten alloy into a tubular mold of fused alumina, the sidewalls of which were heated 7 tea temperature of about 1400? 0, just prior to pouring.

The bottom of the mold consisted of a substantially planar water-cooled-copper body whose temperature was maintained at about 20 C. whereby substantially all the superheat and latent heat of the molten metal was extracted through said cooled copper bottom durin'gsolidification' to provide an ingot having an as-cast grain structuresubstantially consisting of a plurality of elongated columnarg'rains extending upwardly from'the copper bottom, sub--. stantially as shown and disclosed'injthe previously referenced co-pending application. After cooling, the 'ingotwas cut into a 'plurality of slabs, each slab having a pair of parallel faces, said faces extending in a direction'parallel to the mean direction of the longitudinal axes of the columnar grains, i.e., sub- I stantially parallel to the direction of flow of the majority of the heat extracted duringsolidification of the casting or, in other words, substantially perpendicular to the copper bottom of the mold. f i i These slabs were reduced by'a plurality of rolling passes during which passes the longitudinal axes of the columnar grains were maintained in a direction substantially parallel to the rolling plane and to the rolling direction. 'After about 70% reduction in thickness of these slabs had been accomplished by cold rolling, certain of these cold rolled bodies were annealed at various times and temperatures and magnetic torque tBStStWEl'd performed on specimens cut therefrom. The-results of these torque tests are reproduced infthefollowing table.

Table .I 1

Time (Hours) 'Peak Ratio Anneal Temp, 0. I

55000000 XXX P9999995 lqqtommcm GJQOPFWOQN From the foregoing, as will be appreciated by those tion was annealed for 8 hours at 1000" C. and certain of these annealed bodies were then subjected to a'further 50% reduction in'thickness in cold rolling and the re mainder of these bodies were then subjected to 'a further 70% reduction in thickness by cold rolling. These coldrolled bodies were annealed and magnetic torque tests were per-formed upon specimens cut therefrom.

Table II Maximum Percent Reduction Anneal Time Torque, Peak in Thickness Temp., (Hours) H=1,000 Ratio O. (Ergs per From the foregoing test results, it is apparent that the cube texture orientation of these materials may be retained through cold working or recrystallization heat treatments and contrary to that which might reasonably be expected, does not change upon recrystallization to a (110) [001] orientation in any substantial degree.

From this it is apparent that ingots having an as-cast grain orientation consisting of columnar grains as previously disclosed may be reduced from quite heavy as-cast sections to sheet or strip-like material having a recrystallized cube texture without any substantial restriction as to the initial thickness of the ingot. From this, it may be seen that large ingots may be so-cast, rolled and heat treated to form cube texture or strip without having to first reduce said ingots into relatively thin slabs prior to rolling.

For a specific example, assume it is desired to prepare nominal 0.012 inch thick cube texture sheet material of 3% silicon-iron from, for example, a 24" x 24" square, cross section ingot of any appropriate length having substantially all of its as-cast grains in the form of elongated columnar grains Whose longitudinal axes extend substantially lengthwise of said ingot.

According to my invention this may be accomplished, for example, in the following manner. The ingot is heated to a temperature of about 1100 C. and rolled without reheating in a plurality of passes to elfect about a 90% reduction in thickness to an intermediate slab of about 1.4" thick. During this and subsequent rolling operations the ingot is preferably rolled with its length parallel to the rolling plane and the rolling direction.

This intermediate slab is annealed by heating to about 1000 C. and holding at that temperature for about four hours. The 1.4" thick slab is then cold reduced about 70% by rolling to about 0.42" thick, annealed to a temperature of about 1150 C. and permitted to furnace cool to produce a strong cube texture.

This cube texture material is then further reduced in a plurality of cold rolling steps with interspersed anneals. Preferably the annealing steps are accomplished after a substantial amount of cold reduction, for example, about 50 to 70% has been accomplished. For purposes of this specific example, the 0.42" thick sheet material having cube texture is then cold reduced about 70% to about 0.126" thickness, annealed, for example, for about 8 hours at about 1000" C., cold reduced about 70% to about 0.38" thickness, annealed for about 8 hours at about 1000 C., cold reduced about 70% to about 0.012 thick sheet material and annealed for about 8 hours at about 1000 C. to eifect recrystallization of the cold worked crystal structure to cube texture.

It will be appreciated that the foregoing specific illustration involving an ingot having a 24" x 24" square cross-section is exemplary only and that other specific sizes and cross-sectional configurations may be equally Well employed with appropriate adjustments in the schedule of rolling reductions. Furthermore, the recrystallization of the cold worked structure of these materials may be efiected over annealing temperature range of from about 800 C. to over 1200" C. for intervals of time ranging from a few minutes to several, i.e., 8 or more hours, depending primarily upon the particular temperature employed.

It will be appreciated that the specific amounts of rolling reduction and annealing times and temperatures may be varied. For example, it would appear desirable that the amount of cold reduction of thickness before annealing be at least 40% in every case and for commercial considerations may be as large as is convenient. Further, the annealing times and temperatures required after each cold reduction would appear to be quite flexible, ranging from about ,6 hour at about 1200 C. to as much as 8 hours or more at about 800 C., so long as the cold worked structure is recrystallized.

From the foregoing, it will be apparent that I have invented a method for producing cube texture soft magnetic sheet material composed of alloys consisting of at least 92% iron and up to 5% silicon, up to 5% molybdenum and up to 8% aluminum, by directly processing large commercial size ingots.

What I claim as new and desire to secure by Letters Patent of the United States is:

A method for producing polycrystalline sheet-like metal having the body-centered cubic crystal lattice form by rolling and heat treating in which a majority of the grains thereof have the cube texture preferred orientation with respect to the rolling direction and rolling plane of said sheet comprising the steps of rolling a grain oriented ingot composed of a binary alloy selected from the group consisting essentially of up to about 5% silicon, balance substantially all iron, up to about 8% aluminum, balance substantially all iron, and up to about 5% molybdenum, balance substantially all iron to effect a reduction in thickness of at least 40% and form an elongated slab-like body, said ingot being characterized in its as-cast state by being substantially entirely composed of a plurality of elongated columnar grains, the longitudinal axes of which are substantially parallel, said longitudinal grain axes being maintained substantially parallel to the rolling direction and the rolling plane during the rolling operations, annealing said slab at a temperature of from about 800 C. to 1200 C. for a period of time sufiicient to efi'ect recrystallization of the cold-worked metal, cold rolling said annealed slab to efiect at least a 40% reduction in thickness and form an elongated sheet-like body, annealing said sheet-like body at a temperature of from about 800 C. to 1200 C. for an interval of time suificient to elfect recrystallization in which a majority of the grains have a cube texture preferred orientation, cold rolling said annealed cube texture sheet-like body to effect at least a 40% reduction in thickness, and annealing said sheet-like body at a temperature of from 800 C. to 1200 C. for an interval of time suflicient to effect recrystallization of a majority of the grains of said sheetlike body into the cube texture preferred orientation.

References Cited in the file of this patent UNITED STATES PATENTS 2,112,084 Frey et a1 Mar. 22, 1938 2,307,391 Cole et al. Jan. 5, 1943 2,700,006 Dunn Ian. 18, 1955 FOREIGN PATENTS 610,440 Great Britain Oct. 15, 1948 

